Exogenous metabolism is plainly a process of quite a different order from that of endogenous, or tissue metabolism. The latter involves oxidation, while the former consists essentially of a series of hydrolytic cleavages which result in a rapid elimination of the proteid-nitrogen as urea. In this conception of exogenous katabolism, we have essentially the same viewpoint as was previously taken in attempting to explain how excess of proteid food can be so quickly decomposed, and its nitrogen removed from the body. Whether the hydrolytic cleavage is accomplished solely by trypsin and erepsin, whether it takes place only in the intestine and in the liver, or whether other glands and tissues are involved, is at present immaterial; the essential point is that we have in the body a variety of proteid katabolism, quite different from true tissue katabolism, the extent of which is dependent primarily upon the amount of proteid food consumed. The process involved is one which aims at the rapid removal of the proteid-nitrogen as urea; without incorporation of the absorbed proteid, or its decomposition products, either as an integral or adherent part of the tissue proteid. Hydrolytic cleavage is eminently fitted to accomplish this with the least expenditure of energy, while the carbonaceous residue of the proteid thus freed from nitrogen can be transformed into carbohydrate, or directly oxidized as the needs of the body demand. As one considers these views so admirably worked out by Folin, the question naturally arises, if the real demands of the body for proteid food will not be adequately met by the quantity necessary to satisfy the true tissue metabolism? We may well believe, with Folin, that “only a small amount of proteid, namely, that necessary for the endogenous metabolism, is needed. The greater part of the proteid furnished with so-called standard diets, like Voit’s, _i. e._, that part representing the exogenous metabolism, is not needed; or, to be more specific, its nitrogen is not needed. The organism has developed special facilities for getting rid of such excess of nitrogen, so as to get the use of the carbonaceous part of the proteid containing it.” In endogenous metabolism, we have a steady, constant process quite independent of the amount of proteid food, and absolutely indispensable for the maintenance of life. So far as we know at present, its representative creatinin is, for a given individual, the same in amount during fasting as when a rich, meat-free, proteid diet is taken. The one factor that seemingly determines the amount of creatinin eliminated is the weight of the individual, or more exactly the weight of the true tissue elements of the body, as distinct from fat or adipose tissue. Endogenous or tissue katabolism obviously calls for a certain quantity of proteid to maintain equilibrium, but this is small in amount as compared with the usual intake of proteid foods. The average man, with